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<title>mq_receive</title>
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The Single UNIX &reg; Specification, Version 2<br>
Copyright &copy; 1997 The Open Group

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<h4><a name = "tag_000_007_758">&nbsp;</a>NAME</h4><blockquote>
mq_receive - receive a message from a message queue
(<b>REALTIME</b>)
</blockquote><h4><a name = "tag_000_007_759">&nbsp;</a>SYNOPSIS</h4><blockquote>
<pre><code>

#include &lt;<a href="mqueue.h.html">mqueue.h</a>&gt;

ssize_t mq_receive(mqd_t <i>mqdes</i>, char *<i>msg_ptr</i>, size_t <i>msg_len</i>,
    unsigned int *<i>msg_prio</i>);
</code>
</pre>
</blockquote><h4><a name = "tag_000_007_760">&nbsp;</a>DESCRIPTION</h4><blockquote>
The
<i>mq_receive()</i>
function is used to
receive the oldest of the highest priority message(s)
from the message queue specified by
<i>mqdes</i>.
If the size of the buffer in bytes, specified by the
<i>msg_len</i>
argument,
is less than the
<i>mq_msgsize</i>
attribute of the message queue, the function fails and returns an error.
Otherwise, the selected message is removed from the queue
and copied to the buffer pointed to by the
<i>msg_ptr</i>
argument.
<p>
If the value of 
<i>msg_len</i>
is greater than {SSIZE_MAX}, the result is implementation-dependent.
<p>
If the argument
<i>msg_prio</i>
is not NULL, the priority of the selected message is stored
in the location referenced by
<i>msg_prio</i>.
<p>
If the specified message queue is empty and O_NONBLOCK
is not set in the message queue description associated with
<i>mqdes</i>,
<i>mq_receive()</i>
blocks until a message is enqueued on the message queue
or until
<i>mq_receive()</i>
is interrupted by a signal.
If more than one thread 
is waiting to receive a message when a
message arrives at an empty queue and the Priority Scheduling
option is supported,
then the thread 
of highest priority that has been waiting the longest
will be selected to receive the message.
Otherwise, it is unspecified which waiting thread 
receives the message.
If the specified message queue is empty and O_NONBLOCK
is set in the message queue description associated with
<i>mqdes</i>,
no message is removed from the queue, and
<i>mq_receive()</i>
returns an error.
</blockquote><h4><a name = "tag_000_007_761">&nbsp;</a>RETURN VALUE</h4><blockquote>
Upon successful completion,
<i>mq_receive()</i>
returns the length of the selected message in bytes
and the message is removed from the queue.
Otherwise, no message is removed from the queue,
the function returns a value of -1, and sets
<i>errno</i>
to indicate the error.
</blockquote><h4><a name = "tag_000_007_762">&nbsp;</a>ERRORS</h4><blockquote>
The 
<i>mq_receive()</i>
function will fail if:
<dl compact>

<dt>[EAGAIN]<dd>
O_NONBLOCK was set in the message description associated with
<i>mqdes</i>,
and the specified message queue is empty.

<dt>[EBADF]<dd>
The
<i>mqdes</i>
argument is not a valid message queue descriptor open for reading.

<dt>[EMSGSIZE]<dd>
The specified message buffer size,
<i>msg_len</i>,
is less than the message size attribute of the message queue.

<dt>[EINTR]<dd>
The
<i>mq_receive()</i>
operation was interrupted by a signal.

<dt>[ENOSYS]<dd>
The
<i>mq_receive()</i>
function is not supported by this implementation.

</dl>
<p>
The
<i>mq_receive()</i>
function may fail if:
<dl compact>

<dt>[EBADMSG]<dd>
The implementation has detected a data corruption problem with the message.

</dl>
</blockquote><h4><a name = "tag_000_007_763">&nbsp;</a>EXAMPLES</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_007_764">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_007_765">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="mq_send.html">mq_send()</a></i>,
<i><a href="mqueue.h.html">&lt;mqueue.h&gt;</a></i>,
<i><a href="msgctl.html">msgctl()</a></i>,
<i><a href="msgget.html">msgget()</a></i>,
<i><a href="msgrcv.html">msgrcv()</a></i>,
<i><a href="msgsnd.html">msgsnd()</a></i>.
</blockquote><h4>DERIVATION</h4><blockquote>
Derived from the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995)
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Copyright &copy; 1997 The Open Group
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